While Wikipedia is typically a great reference, it's wrong on that one. ROT stands for "rate of turn". There is not only rate 1 turns but rate 2 and 3 and I guess you could continue past that. Rate 2 is a 6 degree/sec turn and rate 3 would be a 9 degree/sec turn. Half rate turns are sometimes used as well; 1.5 degree/sec.
imgur: the simple image sharer
Also of note, if you search just ROT in wikipedia it comes up correctly.
ROT - Wikipedia, the free encyclopedia